European Patent No. EP 1 502 640 A1 describes a filter element having an inlet area and an outlet area, and a plurality of inlet channels and a plurality of outlet channels, the inlet channels having a hexagonal cross section and the outlet channels having a square or rhomboid cross section, and the inlet channels and outlet channels being separated by a filter wall made of an open-pored material.
Filter elements are often made of a ceramic material and are usually manufactured by extrusion. This means that the filter element blank is a prismatic body having a plurality of channels running in parallel. The channels of a blank are initially open at both ends.
To allow the exhaust gas that is to be cleaned to flow through the walls of the filter, some of the channels are sealed at the rear end of the filter element, while other channels are sealed at the front end of the filter element. This forms two groups of channels, namely so-called inlet channels, which are closed at the rear end, and so-called outlet channels, which are closed at the beginning of the filter element.
There is a flow connection between the inlet channels and the outlet channels exclusively through the porous walls of the filter element (hereinafter: filter walls) so the exhaust gas is only able to flow through the filter element by flowing out of the inlet channels and into the outlet channels through the walls of the filter element.
With the conventional filter element, soot particles become deposited on the upstream surface of the filter wall over a period of time, These soot particles cause a reduction in permeability of the filter wall and consequently cause an increase in the pressure drop which occurs in passage of the gas flow through the filter wall. Accordingly, the so-called “exhaust backpressure” increases. If it exceeds a certain value, the filter is regenerated by burning off the deposited soot particles. Heat is released in this process, resulting in an increase in temperature in the filter element.
The larger the filter area of the honeycomb body, the longer the regeneration intervals may be. Honeycomb bodies with a high cell density, i.e., with small channel diameters, have a high volume-specific filter area. However, the channels on the inlet side must not be too small because otherwise there is the risk of blockage of the channels due to ash or soot particles. Furthermore, if the pressure drop caused by the flow through the filter channels is too great in relation to the total pressure drop of the filter, there is the risk of uneven loading. This is a disadvantage in particular when this high ratio is caused by the channels on the outgoing flow end of the filter because in this case the flow passes through the filter wall in the rear area of the filter, where a greater amount of soot is deposited. During regeneration, the highest temperatures occur in this part of the filter anyway. This effect is further potentiated by a large amount of deposited soot. In the case of filter elements made of cordierite, this risk is particularly great because cordierite has a comparatively low specific thermal capacity and therefore very high temperatures may occur locally during oxidation of soot deposits. Consequently, under unfavorable circumstances during regeneration, such high temperatures may occur within the filter element that the thermal stability of the cordierite is no longer ensured. This relationship has so far prevented the use of cordierite filter elements in passenger vehicles.